It has been proposed to measure an ion concentration by measuring potential response or current response of a carbon material or the like used as a conductive material on which a redox functional layer is coated. However, in this case, the potential or current response of the electrode is generated on the boundary between the different substances. The adhesion between the different substances is therefore a problem. In addition, since an ion concentration is measured in an aqueous solution, differences between the degrees of swelling of the materials used cause the separation of the materials or the occurrence of pin-holes. It is thus difficult to form an ion sensor having stability for a long time.